1. Field of the Invention
The present invention relates to a method of manufacturing a rotor for a screening apparatus and a rotor structure for a screening apparatus. The rotor structure of the invention is particularly suitable for screening fibre suspensions of the pulp and paper industry. The apparatus according to the invention relates to a novel rotor construction, and especially to a novel means of fastening a turbulence element on the rotor surface.
2. Description of Related Art
The screening apparatus used nowadays in the pulp and paper industry is almost without exception a pressurized screening device i.e. a so-called pressure screen into which the pulp to be screened is introduced in a pressurized state. The most popular pressure screens comprise a stationary screen cylinder and a rotating rotor in cooperation therewith. The purpose of the screen cylinder is to divide the fresh pulp or the fibre suspension entering into the screening cavity where the rotor rotates into an acceptable fibre fraction called the accepts, and a rejectable fibre fraction called the rejects. The screen cylinder as well as, naturally, the rotor are located inside a screen housing having ducts for both the fresh fibre suspension, the accepts, and the rejects. Normally, the inlet duct or inlet for the fibre suspension is at one end of the screen housing, whereby the rejects outlet is at the opposite end of the housing. The accepts outlet is in communication with the accepts cavity, which is positioned at the opposite side of the screen cylinder in relation to the screening cavity. The purpose of the rotor is to create turbulence, and positive and negative pressure pulses in the fibre suspension to be screened. This purpose is achieved by providing the rotor with specific turbulence elements.
At this stage it should be understood that screening devices whose screen cylinder is rotary, and the means creating turbulence and pressure pulses is stationary, are also known, though more seldom used. The word ‘rotor’ is supposed to cover also this kind of turbulence creating means, as they can be said to rotate in relation to the screen cylinder. Also it should be understood that the term ‘screen cylinder’ covers all screening means having openings, i.e. holes or slots, for instance, and having a rotationally symmetric shape. Thus also conical or frusto-conical shapes are covered, and also known from prior art.
The pressure screen is most often positioned such that its shaft is in an upright position. However, the pressurization of the fibre suspension makes it possible to position the shaft of a pressure screen in any direction including a horizontal direction. Due to the pressurized feed of the fibre suspension, it may be introduced into a pressure screen to the top, to the bottom or to the centre region thereof.
The pressure screens may also be divided into two groups based on the direction of the accepts flow through the screen cylinder. When the accepts flow is radially outwardly, the screen is called an outflow screen, and when the accepts flow is radially inwardly, the screen is called an inflow screen.
In accordance with the prior art there are, in principle, two different types of rotors, which are commonly used in the pulp and paper industry and the intention of which, as known, is to maintain the screen surface clean, in other words to prevent blockages of perforations in the screen surface, and to maintain sufficient turbulence in the screening cavity containing fresh i.e. non-screened fibre suspension. The rotor types may be called an open rotor and a closed rotor. An example of an open rotor is disclosed in U.S. Pat. No. 4,193,865 in which the rotor is arranged inside a cylindrical, stationary screen cylinder. The rotor comprises a concentric shaft and a number of turbulence elements in the form of foils extending close to the surface of the screen cylinder. Each foil is supported on the shaft by means of a pair of arms extending through the cavity, which contains fresh pulp when the screening apparatus is in operation. The foils of the above-mentioned patent form an angle with the shaft of the rotor and the axis of the screen cylinder. However, the foils may also be arranged parallel to the axis. While the foil, or the fibre suspension in relation to the foil, is moving, the leading surface of the foil subjects the screen surface to a positive pressure pulse, which pushes acceptable fibres through the screening openings, and the trailing surface of the foil subjects the screen surface to a negative pressure pulse for opening the perforations of the screen surface or, rather, for preventing the fibres from accumulating on the screen surface and from blocking the screening openings by means of creating a back flow from the accepts cavity to the screening cavity.
An example of the other rotor type i.e. the closed rotor has been discussed, for instance, in U.S. Pat. No. 3,437,204, in which the rotor is a substantially cylindrical closed body positioned inside a screen cylinder. The rotor surface is provided with turbulence elements, i.e. protrusions, which, in this example, are almost hemispherical in form. In this kind of an apparatus, the fresh fibre suspension is fed between the rotor and the screen cylinder, whereby the protrusions of the rotor, the so-called bumps, in this case, create turbulence and pressure pulses towards and away from the screen cylinder. In other words, the leading surface of each bump pushes the pulp towards the screen cylinder and the trailing surface of the bump induces a suction pulse that draws the fibre accumulations from the openings of the screen cylinder. Most often the closed rotor surface is cylindrical. In a broader sense, also rotationally symmetrical rotor surfaces may be discussed, as there are rotors having a frusto-conical shape or a dome shape. Additionally, there are also rotors not literally having a rotationally symmetrical shape. One such option is a so-called S-rotor, which is formed of two identical cylinder halves attached to each other such that two radially, or substantially radially, arranged surfaces join the half-cylindrical surfaces. Also, there are rotors formed of a number of planar, possibly rectangular, members arranged to form an annular surface. Further, there are rotors, which are formed of a number of discs attached one on top of the other. The discs have an ellipsoidal outer surface, and the discs are positioned such that the foci of two adjacent discs are not situated in the same plane running along the rotational axis of the rotor.
As to the shape of the turbulence elements arranged on the surface of a closed rotor there is a huge number of different alternatives. A first alternative is a turbulence element, which is a more or less hemispherical bump, as already discussed above. A second alternative is formed of an axially or spirally extending ridge, which still has a rounded top surface. A third alternative is formed of a grooved rotor surface where the groove is formed of a bottom surface, an inclined side surface and a side surface perpendicular to the envelope surface of the rotor. The groove is either axially oriented or spiral. Depending on the width of the bottom surface one could also call the rotor surface not grooved but ridged. A fourth alternative is formed of a protrusion, which, in a way, resembles the above ridged rotor except that the ridge is cut such that the length of a protrusion is of the order of 50-200 mm. This protrusion type has a number of variations. The leading surface of the protrusion may be perpendicular to the rotor surface or inclined; it may also be axially oriented or inclined in either direction. The protrusion may, or may not, have a top surface either parallel to the rotor envelope surface or inclined in either direction. The protrusion also has a trailing surface which is either inclined or perpendicular to the rotor surface. Thus one has four variables, each having several options, whereby the number of possible alternatives for the shape of a protrusion is very high. And finally, as the fifth alternative, where the surfaces (leading, top and trailing surfaces) of the protrusion may be arranged to be smoothly changing whereby they form a curved surface being formed of several sections each having (possibly) a different radius. In fact, the fifth alternative is formed by combining the foil of an open rotor with a closed rotor, as here the foil has been (with possibly minor modifications to the surface facing away from the screen surface) attached on the surface of the rotor. Thus, when taking into account the above mentioned surface options, though they may also comprise planar sections, the number of possible shapes of the turbulence elements grows even higher.
Yet one more rotor type may be mentioned. It is, in a way, a combination of an open rotor and a closed rotor, as the rotor has both types of turbulence elements i.e. both protrusions, which are fastened from their bottom on a closed rotor surface, and foils being attached by means of short arms on the rotor surface, or even by means of longer arms on the rotor shaft, whereby the rotor can be called either a partially closed or a partially open rotor.
U.S. Pat. No. 6,029,821 discloses a rotor structure for screening fiber suspensions. The rotor is comprised of a rotor body comparable to s rotor shaft to which radial arms for rotor foils have been fitted. The foils are arranged at a distance from the rotor body. The foils have been attached to the arms by means of screws extending through the foil from the foil surface facing the screen cylinder. The radially inner end of the arms is inserted in an opening in the rotor body, and welded therein.
U.S. Pat. No. 4,663,030 discloses a disk rotor for paper making stock screens. The interior of the screen cylinder is for the most part open, as the rotor has been formed of a shaft and a planar disk arranged at the upper end of the shaft. The disk extends close to the screen cylinder inner surface such that rotor foils may be attached on the outer circumference of the disk and rotate in close proximity of the screen cylinder. The foils are attached on the disk by means of bolts extending through the foils from the foil surface facing the screen cylinder. The disk surfaces to which the foils are to be fastened are flattened.
EP-A2-1 143 065 discloses a screening rotor formed of a rotor body (corresponding to a rotor shaft), and foils attached by means of arms to the rotor body. The foils are always arranged at a distance from the rotor body